1. Field of the Invention
The present invention relates to a phase shift interferometer used for optical measurement and tomographic image measurement based on optical coherence tomography of optical materials and optical parts.
2. Description of the Related Art
As noninvasive, high-resolution measurement means for measuring the shape of an object in an industrial field or the like, measurement using interference of light is conventionally performed using a spectral interferometer. Such a spectral interferometer is used to measure a phase of light in spectral domain.
In the spectral interferometer, propagation light emitted from a light source is split into two paths. Sample under measurement is placed in one of the paths and light transmitted through the sample is probe light. Sample is not placed in the other path and light propagating through the path in which the sample has not been placed is reference light.
The probe light and the reference light are combined to cause interference between the probe light and the reference light. Characteristics of the sample are obtained from the results of this interference.
Here, a wavelength-tunable light source which oscillates at a single wavelength is used for the light source in the spectral interferometer. Wavelength (or frequency) of the light (propagation light) emitted by the light source is swept and intensity of the interference light is measured using the wavelength (or the frequency) as a parameter. Through this measurement, a function for wavelength as a parameter is obtained, and change in phase of the light with transmission or reflection of the sample is obtained. A spectral phase can be obtained from the change in the phase of the light.
Characteristics of optical materials, optical components or the like can be evaluated by obtaining chromatic dispersion from the spectral phase obtained as described above.
Further, a spatial distribution of an optical medium in light propagation direction can be obtained by performing inverse Fourier transform of the spectral phase. From this spatial distribution, for example, a tomographic image in depth direction of the sample can be obtained.
When the spectral phase is measured from the interference light, the sign of the phase of the light cannot be determined by measuring only one of cosine (cos) component and sine (sin) component. Therefore, a mirror image of spectral phase inverted around the origin of the frequency axis against the spectral phase which should be obtained in reality is obtained. As a result, it cannot be determined whether the chromatic dispersion increases or decreases or whether the distance in the depth direction increases or decreases.
Therefore, to avoid the generation of the mirror image spectrum on the frequency axis as described above is, it is important to measure both of the cos component and the sin component, namely, two orthogonal components such that the sign of the phase of the light can be determined.
An interferometer capable of measuring two orthogonal components described above includes a phase shift interferometer. A configuration of a general phase shift interferometer is disclosed in “Phase shifting interferometry” in “Optical Shop Testing,” by H. Schreiber, J. H. Bruning and J. E. Greivenkamp, edited by D. Malacara (3d ed.) chap. 14, pp. 547-666 (J. Wiley & Sons, New Jersey, 2007), which is hereinafter referred to as Non-Patent Document 1. In the phase shift interferometer described in Non-Patent Document 1, a phase shifter may be provided in one of two paths constituting the phase shift interferometer. Phase shift generated in this phase shifter is switched to “0” and “π/2” (in radians), and two orthogonal components of the cos component and the sin component are measured.
Further, in Non-Patent Document 1, a PZT (lead titanate zirconate: piezoelectric material) transducer is mounted as the phase shifter on a mounting portion of a mirror or a lens. As the PZT (lead titanate zirconate: piezoelectric material) transducer is mounted on the mounting portion of the mirror or the lens, the mirror or the lens is displaced to switch a phase shift amount for light propagating through a path on one side.
Furthermore, a plurality of algorithms for obtaining, when a non-interference component that is a background component is included in a measured interference waveform, two orthogonal components by removing this non-interference component from the interference waveform to extract the interference component are described in Non-Patent Document 1.
Further, there is an example in which a method of switching a phase shift amount shown in the phase shift interferometer and measuring two orthogonal components is applied to a spectrum interferometer. For example, Japanese Unexamined Patent Application, First Publication No. 2001-059714, which is hereinafter referred to as Patent Document 1 discloses measurement of two orthogonal components of a spectral phase using a wavelength scanning phase shift interferometer. The wavelength-swept phase shift interferometer described in Patent Document 1 is configured as described below.
A light beam emitted from a light source including an external cavity LD (Laser Diode: semiconductor laser) is converted to collimated light by a telescope, and this collimated light is emitted to a Michelson interferometer.
In the Michelson interferometer, the incident collimated light is split into irradiation light and reference light, the irradiation light is radiated to an object under measurement and the reference light is radiated to a PZT mirror functioning as a phase shifter. The irradiation light radiated to the measured object is reflected from a surface of the measured object and converted to object light.
Also, the reference light reflected from the PZT mirror and the object light emitted by the measured object are reflected by a beam splitter, transmitted through a polarizer and then superimposed on a photoelectric surface of a CCD (Charge Coupled Device) in a CCD camera. Through this superposition, an interference signal of the reference light and the object light is detected on the photoelectric surface.
The PZT mirror is controlled to be in a position corresponding to arbitrary phase and an interference spectrum is measured in such a phase. Also, after the measurement of the interference spectrum at the phase is completed, the PZT mirror is controlled to be in a position corresponding to a phase different from the above phase, and measurement of the interference spectrum is performed at this phase.
The interference spectra in different phase components are measured for each phase component by repeatedly performing a process of changing the position of the PZT mirror and measuring the interference spectrum in different phases as described above. Here, determination of the phase value is performed only in the center wavelength in a wavelength range in one sweep when the wavelength is swept, and a phase value in other wavelengths in the same wavelength range is considered to be equal to the phase value in the center wavelength.
Further, when a tomographic image in optical coherence tomography is obtained using the phase shift spectral interferometer, mirror image data in the tomographic image can also be eliminated (e.g., see “Full range complex spectral optical coherence tomography technique in eye imaging,” by M. Wojtkowski, A. Kowalczyk, R. Leitgeb and A. F. Fercher, OPTICS LETTERS vol. 27, no. 16, pp. 1415-1417 (2002), which is hereinafter referred to as Non-Patent Document 2).
Further, two orthogonal components can be obtained using a heterodyne optical spectrum analyzer having a configuration of an optical beam splitter which emits interference components corresponding to different phase components in parallel from three respective output ports instead of using the phase shifter (e.g., see PCT International Publication No. WO 2004/005974, which is hereinafter referred to as Patent Document 2). Simultaneously obtaining two orthogonal components in heterodyne interference through a parallel process using interference components emitted in parallel from three output ports is described in Patent Document 2.
However, in the swept-wavelength phase shift interferometer disclosed in Patent Document 1, approximation is performed so that swept wavelength is equal to the center wavelength. Therefore, a configuration based on a scheme of obtaining the predetermined phase values only in the center wavelength, with at least three predetermined phase values in the central wavelength corresponding to phase values in other scanning wavelengths, is disclosed in Patent Document 1.
In a scheme of Patent Document 1, the chromatic dispersion with the propagation of the path in the interferometer and the reflection from a measured object is small enough not to affect measurement accuracy and can be neglected, and a correct function is performed under a condition that an approximation that the phase of the propagation light linearly changes with respect to the frequency is satisfied.
On the other hand, when the chromatic dispersion cannot be neglected relative to measurement accuracy, only low-precision measurement can be performed, and when the chromatic dispersion of the measured object is characteristics to be measured, the phase does not linearly change with the frequency and therefore the chromatic dispersion cannot be precisely measured.
In other words, when the measurement of the chromatic dispersion is intended, the interferometer having the configuration disclosed in Patent Document 1 cannot be used because of measurement conditions of such a scheme.
Further, the interferometer disclosed in Patent Document 1 is constructed with a free-space optical system defining an interference path to cause interference of light in air as can be seen from the configuration illustrated in FIG. 1 of Patent Document 1, which is an obstacle to downsizing an apparatus.
Here, when an interference path is configured of an optical fiber for the purpose of downsizing and easy configuration, it may be considered that the phase does not linearly change with the frequency because of chromatic dispersion in the optical fiber, and measurement accuracy of the chromatic dispersion of a measured object is degraded.
Therefore, it is necessary for the chromatic dispersion in the optical fiber to be suppressed so as not to affect the measurement accuracy by shortening the length of the optical fiber.
However, if the optical fiber is shortened so as not to affect the measurement accuracy, the length of the optical fiber to reach a diagnosis part in optical coherence tomography cannot be secured.
Further, it is necessary for a wavelength sweep range (a wavelength bandwidth in which a wavelength is swept, i.e., a wavelength range for measurement) to be narrowed to a range in which approximation of the phase linearity to the frequency is possible. Since the wavelength sweep range is narrowed in this way, only an interference signal in a narrow range is obtained and resolution of the tomographic image is degraded.
Furthermore, in the interferometer disclosed in Patent Document 1, a PZT mirror is used to constitute the phase shifter, the PZT mirror is fixed to a position corresponding to each of predetermined phase values, and wavelength sweep is repeatedly performed to measure an interference spectrum in each phase value.
Also, the two orthogonal components of interference based on a plurality of phase values are obtained by measuring the interference spectrum in the plurality of phase values by repeatedly performing this sweep.
When the scheme of obtaining the two orthogonal components of Patent Document 1 is applied to an interferometer using optical fibers, position of the PZT mirror is changed due to change in optical path length of the optical fiber with temperature change, and the phase in the interferometers varies while the interference spectrum in a plurality of phase values are being acquired. Therefore, a phase value set for the interferometer differs from a phase value with which measurement is actually performed.
As a result, since orthogonality of the two orthogonal components is impaired, a ripple is generated in chromatic dispersion data or a tomographic image. Measurement accuracy of the chromatic dispersion is greatly degraded due to this ripple such that evaluation is impossible or the tomographic image is disturbed and diagnosis based on the tomographic image cannot be performed.
Similarly, the interferometer disclosed in Non-Patent Document 2 also suffers from the same problems as Patent Document 1 since the PZT minor is moved to change the phase shift amount.
On the other hand, the interferometer described in Patent Document 2 has a configuration of an optical beam splitter that emits interference components corresponding to different phase components from three output ports in parallel. Therefore, since it is unnecessary to use the phase shifter and it is possible to measure interference components of respective different phase components simultaneously, there are no effects of the phase variation as in Patent Document 1.
However, it is necessary to provide a photodetector for each of three output ports. As a result, the number of photodetectors increases, the configuration of the interferometer becomes more complicated as the number of photodetectors increases, it is difficult to reduce the size of the interferometer, and manufacturing cost increases.